The treatment of the surfaces of fillers in order to improve the filler/matrix coupling in thermoplastic and thermosetting composites and in filled rubber systems is known. Thus U.S. Pat. Nos. 4,514,231 and 4,704,414 describe the treatment of fillers with polysulfide silyl ethers having the structure (R.sub.n.sup.1 (RO).sub.3-n Si--(Alk).sub.m --Ar).sub.p).sub.2 (S).sub.x, in particular bis(triethoxysilylpropyl) tetrasulfide. According to DE 2 141 159 and 2 255 577, the addition of the same polysulfide silyl ethers to the mixture of rubber and untreated filler ("in situ surface modification") is also appropriate for the production of highly reinforced and abrasion-resistant rubber articles. A disadvantage of the use of polysulfide silyl ethers for improving the filler/matrix coupling is its expensive production process and the high raw-material costs associated therewith.
U.S. Pat. Nos. 2,736,669 and 2,801,185 describe a process for hydrophobing silicas which is based on the treatment of silicas with alcohols at particular temperatures. But the silica treated in this way does not show an improved reinforcing effect in a rubber matrix.